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Crayfish ‘trapping’ fails to control invasive species - UCL

Crayfish in a trap. Credit: Eleri Pritchard
Crayfish in a trap. Credit: Eleri Pritchard

Despite being championed by a host of celebrity chefs, crayfish ‘trapping’ is not helping to control invasive American signal crayfish, according to new research by UCL and King’s College London.

There have been grave concerns within the science community and amongst conservationists that American signal crayfish are wiping out other species of crayfish across Europe - including Britain’s only native crayfish, the endangered white-clawed crayfish.

In their new study, published in the Journal of Applied Ecology, the researchers find that trapping is ineffective in determining and controlling signal crayfish numbers, as the vast majority of individuals are too small to catch using standard baited traps.

‘Trapping to eat’ American signal crayfish has been promoted as a potential control measure in recent decades, but scientists warn that this method may exacerbate the problem, as it inadvertently incentivises members of the public to spread the species to new habitats and greatly increases the risk of accidental catches of the strictly protected native species.

Co-author and PhD researcher, Eleri Pritchard (UCL Geography), said: “Invasive signal crayfish from the US were introduced to England in the 1970s. Since then they have spread rapidly, displacing native crayfish, impacting fish and damaging ecosystems. While celebrity chefs and conservation charities have, with good intentions, promoted trapping and foraging as a way to control American signal crayfish, our research shows trapping to be ineffective. We are also concerned that trapping risks spreading the fungal pathogen, called crayfish plague, which is lethal to native European crayfish.”

The study took place in an upland stream in North Yorkshire, UK and compared the effectiveness of three methods used for surveying population numbers: baited funnel trapping, handsearching, and a novel ‘triple drawdown technique’, which involves draining a short section of stream in a carefully controlled way and calculating the number of crayfish present, including infants.

To measure effectiveness, scientists compared all methods for determining population size, and for understanding the prevalence of crayfish invasion and the ecological impact.

The triple drawdown technique proved significantly more accurate in determining crayfish populations. Through this method, the researchers identified signal crayfish densities up to 110 per square metre, far exceeding previous estimates for similar streams made by trap sampling.

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